Heroes Chronicles
Outpost 2
Dungeon Siege
Diablo
C & C Series

but the top of my list is Z by the bitmap bros. please make this happen oh mighty GOGsters!!
Hail the All Powerful Blizzard!!!

Kingdom O' Magic (pretty weird but a diamond among adventure games)

Grandia II (I hope GoG make this game work once again, because new OS doesn't support it)

Silver (Awesome Action-RPG from Infogrames)

Blade Runner (Oddly enough this game still works with newer systems, but it's a true gem and deserves a position among GoG games).

No One Lives Forever 1+2 (Superb FPS with a great story)

In no particular order -
* TIE Fighter
* Wing Commander series
* The You Don't Know Jack series of quiz games
* Star Wars Rebellion
* Star Trek: Birth of the Federation

1. SSI Gold Box Series(any)
2. Ultima Series(any)
3. Space Rogue
4. Bards Tale
5. any rpg from the 80s or 90s

1. Frontier Elite 2
2. Ultima 7
3. Homeworld
4.Knights of the old republic
5. Morrowind

Only 5?

Well, let me think of a few...

Magic Candle series.
Dungeon Keeper pack
Omikron: Nomad Soul
Peoples General
King of Dragon Pass

Plenty of others to choose from, but these give me the most pain when it comes to getting them to run on a modern PC (or even semi modern), so I ran with them.

In no particular order...
Grim Fandango (My scythe... I like to keep it next to where my heart used to be. :D )
Vampire The Masquerade Bloodlines
The Witcher Enhanced Edition Director's Cut
Legacy of Kain Blood Omen
Silver

1. Mechwarrior 2 seris
2. Heavy Gear series
3. Xwing series(mainly Tie Fighter)
4. Savage, the old one about being a lion
5. Dune or Dark Reign hard to pick

1) Sid Meier's Alpha Centauri & Alien Crossfire Pack
2) Thief: The Metal Age
4) System Shock 2
5) Vampire: Bloodlines
5) American McGee's Alice

By the by: I own physical, original disc copies of all the above games. But I believe they whole-heartedly belong on GOG to reach a broader and wider audience, past and present. Particularly as they'd be tweaked for modern systems (very important, as most of them are temperamental on modern OS configurations) in their installer, and given the usual GOG makeover.

And the extras would be phenomenal! Having the soundtracks for Bloodlines and Alice in convenient MP3 format alone would be worth the price.

1. Queen: The eYe
2. Goosebumps: Escape From Horrorland
3. Warhammer: Shadow of the Horned Rat / Dark Omen
4. Dust: A Tale of the Wired West
5. The 7th Guest

1.Bioforge
2.I have no mouth and I must scream
3.The Dig
4.System Shock 2
5.Ufo Collectors Edition

It was hard boiling them down to just 5, so I cheated a bit... ;)

#1 Grim Fandango + Day of the Tentacle (+ maybe some other LucasArts classics)
#2 No One Lives Forever 1 + 2
#3 Vampire the Masquerade: Bloodlines
#4 System Shock 2
#5 Thief series

In no particular order, except for Grim Fandango which will always be my favorite game of all times.

1) Freelancer
2) X-Wing Vs Tie Fighter
3) Diablo I & II
4) Age Of Empire II
5) Homeworld

In no particular order:

1. Ultima Series
2. All Looking Glass games
3. LucasArts games including X-Wing etc.
4. Origin games
5. Westwood games such C&C, Lands of Lore etc.